In a die-casting machine, a molten metal (melt) is supplied to a sleeve, injected by a plunger tip sliding in the sleeve into a die cavity communicating with the sleeve, and solidified by cooling to form die castings. Accordingly, an inner surface of the sleeve is melted by the melt, and worn by the sliding plunger tip. With the inner surface of the sleeve damaged by melting and wearing, a melt intrudes into gaps between the sleeve and the plunger tip, increasing the sliding resistance of the sleeve, lowering an injection speed, and deteriorating product quality. When a large amount of a lubricant is used to reduce sliding resistance and to prevent seizure between the sleeve and the plunger tip, impurities such as a gas, etc. are likely introduced into the melt, resulting in deteriorated product quality.
To reduce the melting and wearing of an inner surface of a sleeve, a die-casting sleeve having a composite structure comprising a ceramic-made inner cylinder shrink-fit in a metal-made outer cylinder has conventionally been proposed. For example, JP 7-246449 A discloses a die-casting sleeve comprising an outer cylinder made of a high-strength, low-thermal-expansion metal such as an Fe—Ni—Co alloy, and an inner cylinder made of a ceramic such as silicon nitride, Sialon, etc., which is shrink-fit in the outer cylinder, the high-strength, low-thermal-expansion metal having an average thermal expansion coefficient of 1×10−6/° C. to 5×10−6/° C. at 20-300° C. and 5×10−6/° C. or more at 20-600° C. Such a die-casting sleeve structure provides strong shrink fitting between the outer cylinder and the inner cylinder, resulting in excellent injection stability (melting resistance, wear resistance, heat resistance, melt temperature retention and seizure resistance) and thus stabilized product quality. Also, because the amount of a lubricant can be reduced to about ¼ of a conventional level by the ceramic-made inner cylinder, the generation of gaseous smoke is suppressed, and the sleeve and the plunger tip are provided with a longer life.
JP 9-108811 A discloses a die-casting sleeve comprising an inner cylinder made of ceramics such as silicon nitride, Sialon, etc., which is shrink-fit in an outer cylinder made of an Fe—Ni—Co alloy, to which one or more precipitation-strengthening elements are added, a friction-reducing material such as a fluororesin, graphite or molybdenum disulfide being sandwiched by the outer cylinder and the inner cylinder. The friction-reducing material reduces residual stress in the shrink-fit outer and inner cylinders, preventing the breakage of a tip end portion of the inner cylinder while the sleeve is used, thereby providing the inner cylinder with a long life.
JP 2002-192320 A discloses a die-casting sleeve comprising an inner cylinder fit in an outer cylinder made of hot-die steel such as SKD61, the inner cylinder coming into contact with a molten metal being constituted by a front member on the injection outlet side and a rear member on the rear side of the front member, the front member being made of a high-strength, low-thermal-expansion metal such as an Fe—Ni alloy or its composite material with silicon nitride ceramic particles, and the rear member being made of Sialon.
Any of the above conventional die-casting sleeves has excellent melting resistance, wear resistance, heat resistance and seizure resistance, thereby achieving a long life, because all or part of the inner cylinder is made of ceramics. Nevertheless, the inner cylinders are worn with time, so that they are discarded after use for a predetermined period of time.
To reuse an outer cylinder after the used inner cylinder is detached by heating, the inventors have investigated the shrink-fitting of a new inner cylinder thereto. It has been found, however, that when molding is conducted with such a repaired die-casting sleeve, the sleeve is vibrated as a plunger moves, so that a melt surface is waved in the sleeve, resulting in molding defects.
As a result of intensive research on the causes of defective molding, it has been found that when a new inner cylinder is shrink-fit after the used inner cylinder is detached by heating, a peripheral surface of the outer cylinder has slightly larger deviation from a perfect circle [deviation from roundness, expressed by radius difference of two concentric circles sandwiching a circular shape when they have the minimum gap (JIS B 0621)] than that before detaching, so that the assembling of such a sleeve in a die-casting machine leaves a slight gap between a holding member of the die-casting machine and a peripheral surface of the outer cylinder of the sleeve, resulting in the vibration of the sleeve. The cause of increasing deviation from a perfect circle is not necessarily clear, but it may be presumed that because individual inner cylinders shrink-fit in the outer cylinder do not have the same peripheral surface precision, the outer cylinder receives different distributions of stress from new inner cylinders, so that the outer cylinder is deformed.